Connectivity being the buzzword today, mobile systems and displays demand efficient and longer battery usage and smaller BOM (Bill Of Materials) for a thinner and more compact form factor. Additionally, display quality is the most important performance feature that cannot be compromised even during heavy load current fluctuation, associated large drop-out voltage transition due to it and switching noise from the employed DC-DC converters.
Conventional displays adopt dual output DC-DC drivers where the two outputs are referred as ELVDD (Positive output) and ELVSS (Negative output). The dual outputs supply opposing currents with almost same absolute values and positive and negative voltages related to each display panel load. Each panel has different output current and voltage levels requirements based on the application such as panel size, pixel numbers, display quality, adopted process generation, and the like.
DC-DC drivers typically comprise a boost (step-up), buck (step-down) type DC-DC converter or LDO (Low Dropout) regulators for the positive output and an inverting buck-boost (step-down and step-up) type DC-DC converter for the negative output. Although DC-DC converters have been employed traditionally as it increases the energy efficiency, the power loss from the typically employed 4 nos. switches (2 nos. for positive, 2 nos. for negative) deteriorates the overall efficiency of the driver.
Implementing 4 nos. power switches and 2 nos. controllers for each converter increases total BOM cost and the solution area. Moreover, power switches are very expensive and need more area to get smaller Rds (on) resistance for better efficiency. Each controller is required to be designed carefully to be stable under light or heavy load and even abrupt line or load transients. Additionally, the two DC-DC converters need two expensive and voluminous inductors whose switching operations bring about switching noise/ripple and the different switching noises make them interfere with each other. As a result, it is hard to get a good display quality from dual output DC-DC converters.
There is therefore felt a need for providing cheap and compact drivers with improved efficiency, better power management capability, and having reduced effects on display quality caused by switching operations.